The PIA’s Australian Pump Technical Handbook is a cornerstone text for the Australian pump industry and, in our opinion, a must have for anyone who deals with pumps on a regular basis. In this ongoing series, we feature abridged chapters from the classic book to showcase the various areas covered and to reacquaint readers with the technical aspects of pumps. In this issue, we explore the various factors that should be considered when selecting a centrifugal pump for your specific application.
In order to operate effectively and reliably, a centrifugal pump should be matched to the pumping system and application both hydraulically and mechanically. Installing a mismatched pump can result in ongoing issues throughout the life of the system.
A number of factors should be taken into account in order to select a pump that matches the system and application’s hydraulic requirements. These include:
- System design,
- Quantity – flow rate
- System resistance
- System curve – duty point (rated flow at differential head).
System design – Oversizing or undersizing the pump for the given system should be avoided as it may result in the pump operating outside its allowable operating range. This can cause problems such as circulation issues, vibration, cavitation and loud operation.
Flow rate – The flow rate achieved in a pumping system is the result of the head created by the pump.
System resistance – The system resistance head is the head necessary to overcome the static head and the friction head in the system. The duty point of the pump in a system is the flow at which the head created by the pump is equal to the system resistance head.
Duty point – The key to successful pump selection is to accurately specify the required duty point.
Net positive suction head available
Net positive suction head (NPSH) is the total absolute head at the pump suction minus the vapour head of the pumped fluid.
Determining the NPSH made available by a pump is important for selecting a pump that will operate satisfactorily in your given application.
This relates to the fact that one of the most significant causes of poor pump performance and high maintenance requirements is cavitation.
To prevent cavitation, the system net positive suction head available (NPSHA) must be greater than the pump net positive suction head required (NPSHR), preferably with an appropriate safety margin. (Equations for and sample calculations of these values are available in the Australian Pump Technical Handbook).
If your system is handling cold, clean water, calculations can be simplified using the concept of suction lift.
Pump manufacturers sometimes publish performance curves depicting the suction lift performance of a pump for clear, cold water. However, as with NPSH, allowing a safety margin is recommended.
Matching the pump to these requirements
Having determined the flow rate, total dynamic head to be generated by the pump and the suction lift or NPSHA, you can then select a pump with a performance curve that satisfies all three of these requirements.
Pump manufacturers may present the performance curves of their pumps in a number of different ways:
- Performance presented at a constant speed with different head–quantity curves corresponding to different impeller diameters
- Performance presented at varying speeds with a constant impeller diameter.
Either way, they will also indicate pump efficiency, power use, and the suction lift or NPSH required for the pump’s flow range. The speed (rpm) and input power (kW) required can then be determined by plotting the required flow, total head and suction lift on the performance curves.
However, keep in mind that the input power will usually be given for clear, cold water. Therefore, if the fluid to be pumped is heavier or lighter than water, pumping it will require comparatively more or less power.
Hydraulic selection – moving from ideal to real
The ideal pump for a system would be a pump running at best efficiency point (BEP) and with NPSHA greater than NPSHR by an adequate margin.
In this ideal case, the impeller and volute are operating at pump design conditions and the flow through the pump unit is smooth.
Shock, vibration, energy dissipation, and bearing loading are at a minimum and the pump should have a long, problem-free operating life.
However, this ideal situation is not always possible in reality and a compromise must often be made. This is due to a number of factors:
- A finite number of available pumps – a perfect match for your hydraulic requirements is not always possible because it is simply not economically feasible for pump manufacturers to create a specially designed pump for every possible application. A pump will usually be chosen from the range of commercially available units.
- Specific speed limitations – if the specified duty point is one of high head and low flow rate, impeller geometry limitations may affect the pump selection. This may result in operation near pump minimum continuous flow.
- Net positive suction head – if the NPSHA is limited, the available pump selections and running speeds may be restricted.
- Variable duty requirements – a number of disparate operating conditions may have to be met, precluding an ideal selection.
- Mechanical simplicity/cost/reliability – the ideal selection may be expensive or complicated. Pump selection is often a trade off between efficiency, cost and reliability.
A pump selection is normally considered acceptable if the duty point falls within the rate of 50 to 110 per cent of best efficiency flow rate. Also a selection to the left of BEP is normally preferred to one slightly beyond the BEP.
This allows a greater margin for error in the event that the system designer has overestimated the actual system resistance curve.
In many cases, centrifugal pumps can be successfully operated outside the above recommendations in circumstances where alternative selections are not available or economic.
The pump manufacturer’s advice should be sought in these instances. Nevertheless, it is always preferable to operate a centrifugal pump as close as possible to its BEP.
The effects of poor hydraulic selections
Operating a pump left of BEP can result in problems such as low efficiency, noise and vibration, increased radial loads on bearings due to unbalanced volute pressures, and temperature rise due to dissipated energy created by low efficiency.
On the other hand, operating a pump beyond BEP can result in low efficiency, increased power, noise and vibration, poor suction performance (high NPSHR) and increased radial loads on bearings.
Mechanical factors are also important in the selection of pumps. Many of the different mechanical configurations for pumps have been covered in the articles covering previous chapters of the Australian Pump Technical Handbook.
However, for optimum pump selection, it is useful to briefly examine the effects of system conditions on pump mechanical configuration.
The pump geometry to suit a particular application and its unique set of pumping conditions will depend on the chosen speed and number of stages.
For example, consider the following alternative pump selections for a duty point of 50L/s at a total dynamic head of 150m.
1. Single stage, 1450 rpm. Ns = 239, expected efficiency: 45 per cent, approximate pump outlet size: 100mm, approximate impeller diameter: 720mm. This is not a practical pump selection due to the large impeller diameter required relative to pump size.
2. Single stage, 2950 rpm. Ns = 486, expected efficiency: 68 per cent, approximate pump outlet size: 100mm, approximate impeller diameter: 360mm. This is a reasonable selection with moderate efficiency.
3. Two stage, 2950 rpm. Ns = 818, expected efficiency: 74 per cent, approximate pump outlet size: 100mm, approximate impeller diameter: 260mm. Better efficiency but more complex and expensive.
This example illustrates the way that pump selection is often a trade off between practical design, simplicity, cost and efficiency.
There are three basic types of sealing which are used to seal the pumps where the shaft passes through the pump casing.
These are packed gland seals, mechanical seals and centrifugal seals. Sealless pumps are also becoming more common, particularly in applications where dangerous toxic or corrosive materials are being pumped and leakage cannot be allowed.
The type of seal appropriate will depend on your system and application.
Maintenance and reliability
Greater mechanical complexity may result from attempts to create a more efficient pumping system. In some cases, a well selected multistage pump may also be more reliable than a poorly matched single stage pump.
However, an increase in the number of stages and greater complexity generally provides greater potential for failure.
Aspects of pump construction that affect reliability include:
- Reduced complexity/number of stages (assuming good hydraulic selection),
- Sensible (larger) running clearances,
- Short shaft spans.
It is also worth noting that sometimes components designed for easier maintenance may reduce reliability.
For example easy access to seals and bearings may require a larger shaft span than desirable for reliability considerations.
Effects of pumping conditions
Apart from the purely hydraulic considerations of flow rate and total dynamic head, mechanical construction may be influenced by the effects of static pressure, liquid temperature, suction lift and normal operating point.
High static pressure
High-pressure conditions create the requirement for an adequately designed pressure retaining casing.
Additionally, they can also cause pressure induced axial thrust on the pump shaft, which can lead to rapid bearing failure.
Any pump with a single shaft entry can be affected in this way.
Measures to alleviate this problem include:
- Use of a hydraulically unbalanced impeller producing an opposing thrust.
- Reduced impeller back sealing ring diameter which produces an opposing thrust.
- Oversized thrust bearings.
- Use of double entry between bearings style of pump.
When selecting a pump for operation in high temperature conditions a number of factors must be considered.
These may include:
- Construction materials that can withstand the temperatures they are exposed to.
- Centreline support of pump casing – this allows equal thermal expansion above and below the shaft axis, retaining shaft/coupling alignment, and is recommended at temperatures above 180°C.
- Increased running clearances to avoid internal contact due to differential expansion.
- Shaft sealing methods, such as high temperature mechanical seals.
- Auxiliary coolant requirements for bearings and seals.
- Extremely high temperatures and pressures may require the use of special gasketing and radially split casings only.
Low suction pressure/high suction lift
Operation with suction pressures below atmospheric pressure may result in the ingress of air at the shaft seal(s).Measures to prevent this include:
- Routing a highpressure flush connection to the stuffing box , thus maintaining pressure above atmospheric at the seal/packing
- Use of a suitable mechanical seal
- Use of a suction inducer.
Regular operation at reduced flow
When a single volute pump is operated at flow rates other than BEP, the pressure distribution around the impeller is no longer uniform, producing a radial load on the impeller and shaft which may result in bearing failure and shaft breakage in extreme cases.
If regular reduced flow operation cannot be avoided, consideration should be given to the following:
- Use of a double volute or diffuser style casing.
- Use of a stiffer/stronger shaft assembly.
- Employing a minimum flow bypass line around the pump.
- Use of a variable speed drive to allow reduced flow operations.
Further information and detailed diagrams, equations and schematics can be found in the Australian Pump Technical Handbook, available from the PIA website. In the next edition of Pump Industry, we go beyond selection, to look at the application of centrifugal pumps in a pumping system.